Intravenous delivery of recombinant adenovirus (Ad) vectors for gene therapy is hampered by safety and efficacy problems. We have recently discovered a new pathway that is involved in unspecific sequestration and clearance of commonly used Ad5 and Ad B-group virus Ad11-derived vectors in mice. Rapidly after intravenous administration, Ad5 and Ad11 bind to circulating platelets in a fiber-independent manner. Virus binding results in activation/aggregation of platelets and trapping in micro-blood vessels of the liver, where the virus-platelet aggregates are taken up by Kupffer cells. Ad sequestration in liver can be reduced by platelet depletion prior to vector injection. Platelet depletion increases transduction of liver metastases after intravenous Ad injection, which provides a rationale for studying the mechanism of Ad-platelet interaction with the final goal of constructing oncolytic Ad vectors that are ablated for platelet binding. The goals of this proposal are to identify the structural component/s within the virus capsid that are involved in Ad binding to platelets, and to produce vectors that are ablated for platelet binding. The central hypothesis is that these modified vectors are more efficient in transduction of metastatic tumors after intravenous injection. Although platelet-mediated degradation occurs for both Ad5 and Ad11 vectors, in this proposal, we will focus on modifying Ad11 because of its stronger binding to platelets which might facilitate studying Ad-platelet interactions. Moreover, Ad11-based vector are promising tools for gene therapy. Ad11 infects cells predominantly through CD46, and efficiently transduces important gene therapy targets that are refractory to infection with Ad5 vectors, such as tumor, dendritic, and tissue stem cells. Other advantages of Ad11 over Ad5 vectors include the lower serum prevalence of neutralizing anti-Ad11 antibodies and absence of liver transduction after intravenous vector application. Studies in this proposal will be done in mice that express CD46 in a human like pattern and in mice that contain human platelets. Public Health Relevance: The final aim of this proposal is to produce Ad11 vectors (Ad11?) that are ablated for binding to platelets. The outcome of this study will be relevant for Ad11 mediated tumor gene therapy and vaccination and potentially for the modification of Ad vectors based on other serotypes.